Conventional image-reading devices, such as those disclosed in Japanese unexamined patent application publications Nos. HEI-10-233902 and SHO-62-6574, employing a contact image sensor capable of reading an A3-size original document, for example, are well known in the art. When reading an A4-size original document or other size document having a narrower width than the A3 size, this type of image-reading device shortens the time required to read one line of image data by reading a pixel segment within the original document at normal clock speed and reading an unnecessary pixel segment outside the original document at a fast clock speed for each line, thereby increasing the speed for reading the overall original document. In the example of Japanese unexamined patent application publication No. SHO-62-6574, “bits 1 through 768 are considered data corresponding to the original document, and the shift pulse SP is set to a period of 500 ns when reading this data segment and switched to a period of 125 ns, for example, when reading bits 769 through 2048. Therefore, the image-reading device of the present invention can read a line in 548 ns, which conventionally required 1040 ns.”
However, when increasing the clock speed from the normal speed shown in FIG. 11A to a faster speed shown in FIG. 11B with the technology described above, the clock signal CLK at the contact image sensor connector changes, as shown in FIGS. 11A and 11B, due to an increase in radiation noise in the pixel segment outside the original document and dull waveform of clock caused by impedance in the harness. Since a stable clock level cannot be assured, the clock speed is restricted and cannot be increased to the desired speed.